1. Field of the Invention
The present invention relates to tuners for receiving electromagnetic signals such as radio frequency ("RF") signals. More particularly, the invention concerns a method and apparatus for adaptive self-calibration that quickly compensates for errors resulting from temperature changes and other causes.
2. Description of the Related Art
In the field of radio communications, a radio tuner generally operates to detect and amplify encoded signals received by an antenna coupled to the radio. People use radio tuners in a broad assortment of applications. Radio tuners are frequently found, for example, in commercial band FM or AM radios.
Additionally, radio tuners are frequently used in scanners. When employed in a scanner, a radio tuner incrementally advances through its frequency spectrum while searching for discernable radio signals. When the tuner identifies a radio signal, the tuner locks onto the identified signal until the user performs some action. For example, in a police band scanner, after detecting a radio signal the tuner locks onto the signal until the user initiates a new search for a different signal. Another example of a scanner is the "scan" function in automobile radios, where the tuner only locks onto one radio station for a moment before automatically advancing to the next station. Additionally, scanners are frequently employed in military applications such as radar warning receivers ("RWR"), early warning radar ("EWR"), and other monitoring devices. In military applications, the tuner only locks onto a signal long enough for processing circuitry to determine the nature of the signal, e.g. association with an enemy.
In many scanners, and especially military scanners, speed is critical. These scanners must be able to quickly and accurately detect a radio transmission and then advance to the next signal. In airborne EWR applications, for example, slow signal detection may provide insufficient advance notice to defeat enemy radar.
FIG. 1 illustrates a direct-selection tuner 100. With the direct-selection tuner 100, a digital selector 108 effectively determines the frequency of an output signal 104. The digital selector 108 provides a digital output signal responsive to its input 110. The digital selector 108 may comprise, for example, a mechanical tuning "knob" operated by a human user, or an electronic selector operated by various circuitry (not shown). The digital selector 108 therefore provides a digital signal representative of the desired output signal frequency, in response to the input 110. A digital-to-analog converter ("D/A") 106 converts this digital signal into its analog counterpart, which is fed to a voltage-controlled oscillator ("VCO") 102.
The VCO 102 then generates the output signal 104 having a frequency that is proportional to the analog signal from the D/A 106. The VCO 102 provides the output signal 104 to a down converter 112, connected to an antenna 114. The down converter 112 down-converts signals received from the antenna 114 in accordance with the output signal 104. The down converter 112 provides its output to other circuitry 116, including a demodulator.
Despite its simplicity, the direct-selection tuner 100 provides an adequate level of performance for many applications. A chief advantage of the tuner 100 is its quickness--the tuner 100 can lock onto a desired frequency as quickly as the digital selector 108 can provide the appropriate digital signal. However, the accuracy of the tuner 100 may be insufficient for certain users. If the tuner 100 is subjected to temperature changes, for example, its components may function differently than intended. Thus, a specific input 100 might fail to provide the intended value of output signal 104. This problem is especially vexing in airborne environments, where temperature necessarily fluxuates with altitude changes.
To overcome these problems, engineers have modified the tuner 100 by developing a pre-calibrated tuner 200, as illustrated in FIG. 2. More specifically, for a given temperature there may be a nonlinear relationship between the input 110 and the output signal 104. For instance, if the input 110 is advanced in uniform increments, the output signal 104 might not advance in correspondingly uniform increments. Optimally, when the digital selector 108 provides digital values of 100 units, 110 units, and 120 units, the resulting output signals 104 are 100 MHz, 110 MHz, and 120 MHz, respectively. However, the particular temperature at which the circuitry of the tuner 200 is operating may cause the tuner 200 to provide output signals 104 of 103 MHz, 115 MHz, and 135 MHz, instead.
To compensate for this problem, the pre-calibrated tuner 200 includes a memory 202 such as a programmable read only memory ("PROM"), which performs the functions of a lookup table. The lookup table stored in the memory 202 aids the digital selector 108 in selecting the digital output value. After consulting the lookup table in the memory 202, the digital selector 108 may output a digital value of 97 units, instead of 100 units, to provide the desired output signal 104 of 100 MHz.
Although the pre-calibrated tuner 200 provides a definite improvement over the direct-selection tuner 100, the tuner 200 still may not be completely satisfactory for some applications due to the lengthy calibrations that are required. To operate the tuner 200 at its peak of accuracy, a user must perform two calibration procedures: (1) a module-level linearity calibration, and (2) a system-level calibration.
The module-level calibration involves filling the memory 202 with digital lookup table values corresponding to each combination of temperature and desired output signal 104. In the system-lever calibration, the tuner 200 is operated to amplify a specific test signal that is fed to the antenna 114. This step exposes small variations that different electrical components of the system may experience in actual operation. If the tuner 200 does not lock onto the test signal as expected, the lookup table in the memory 202 is modified as necessary. The system-level calibration steps are repeated for each desired frequency of test signal.
Although the above-mentioned calibration procedures can achieve a reliable level of accuracy, they are time consuming. This may not be problematic for some users, but the required calibration time may be too long for others with sensitive applications; namely, during these lengthy calibration procedures the radio system is rendered inoperative.
In contrast to the tuners described above, some applications use a phase-locked loop ("PLL") tuner. FIG. 3 illustrates a tuner 300 that incorporates a typical PLL. The tuner 300 generally operates to maintain an output signal 302 in a specific phase relationship with an input signal 304. The tuner 300 includes a phase detector 306 to provide an output voltage proportional to the phase difference of the input signal 304 and a feedback signal 308. The phase detector 306 directs its output to a loop filter 310, which controls the dynamics of the tuner 300. The loop filter 310 directs its output to a VCO 312, which generates the output signal 302, providing the output signal 302 with a frequency that is proportionate to the magnitude of the input signal from the loop filter 310. The output signal 302 is fed back through a dividing circuit 314 to produce the feedback signal 308, which is an integer division of the output signal 302.
A down converter 316 uses the output signal 302 to amplify electromagnetic signals received by an antenna 318. The down converter 316 provides the amplified signals to other components 320 of the radio, such as a demodulator and other known circuitry.
Although using a PLL can provide certain advantages, PLL-based tuners are sometimes too slow for some applications. The PLL circuit exhibits an R-C time constant that lengthens the time required for the tuner to lock onto a signal. As a result, the tuner may require hundreds of microseconds to acquire a signal. Therefore, in applications requiring signal acquisition in tens of microseconds or less, PLL-based tuners are not completely satisfactory.